Green flash
Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the sun, or a green ray shoots up from the sunset point. Green flashes are actually a group of phenomena stemming from different causes, and some are more common than others. Green flashes can be observed from any altitude (even from an aircraft). They are usually seen at an unobstructed horizon, such as over the ocean, but are possible over cloud-tops and mountain-tops as well. Explanation The reason for a green flash lies in refraction of light (as in a prism) in the atmosphere: light moves more slowly in the lower, denser air than in the thinner air above, so sunlight rays follow paths that curve slightly, in the same direction as the curvature of the Earth. Higher frequency light (green/blue) curves more than lower frequency light (red/orange), so green/blue rays from the upper limb of the setting sun remain visible after the red rays are obstructed by the curvature of the earth. Green flashes are enhanced by mirage, which increase the density gradient in the atmosphere, and therefore increase refraction. A green flash is more likely to be seen in clear air, when more of the light from the setting sun reaches the observer without being scattered. We might expect to see a blue flash, but the blue is preferentially scattered out of our line of sight and remaining light ends up looking green. With slight magnification a green rim on the top limb of the solar disk can be seen on most clear-day sunsets. However the flash or ray effects require a stronger layering of the atmosphere and a mirage which serves to magnify the green for a fraction of a second to a couple of seconds. Types of green flashes The green flash is actually a group of phenomena, some of which are listed below: The majority of flashes observed are inferior-mirage or mock-mirage ones, with the others constituting only 1% of reports. Some types not listed in the table above, such as the cloud-top flash (seen as the sun sinks into a coastal fog, or at distant cumulus clouds), are not understood. Blue flashes Very occasionally, the amount of blue light is sufficient to be visible as a "blue flash"."The Green Flash, BBC Weather online. Retrieved on 2009-05-07. The term should not be confused with the similar usage of blue flash referring to the blue light seen in nuclear criticality accidents. Green rim is setting behind the Golden Gate Bridge|thumb|150px]] As an astronomical object sets or rises, the light it emits travels through the atmosphere, which works as a prism separating the light into different colors. The color of the upper limb of an astronomical object could go from blue to green to violet depending on the decrease in concentration of pollutants as they spread throughout an increasing volume of atmosphere.Dispersive refraction by webexhibits.org. The lower limb of an astronomical object is always red. A green rim is very thin, and is difficult or impossible to see with the naked eye. In usual conditions a green rim of an astronomical object gets fainter, when an astronomical object is very low above the horizon because of atmospheric reddening,Green and red rims by Andy Young. but sometimes the conditions are right to see a green rim just above the horizon. The following quote describes probably the longest green rim, which at times could have been a green flash, observation. It was seen on and off for 35 long minutes by members of the Richard Evelyn Byrd party from the Little America exploration base in 1934. Often a green rim changes to a green flash and back again during the same sunset. The image below is an illustration of what members of the Richard Evelyn Byrd party from the Little America exploration base may have seen. es of the setting sun|200px|thumb|left]] post."Citation needed for quotation.}} To see green rim on and off for 35 minutes there had to be some mirage present. There's an interesting situation with green rim observing. Green rim is present at every sunset, but it is too thin to be seen with the naked eye. The best time to observe a green rim is about 10 minutes before sunset.Green and red rims by Andy Young. It is too early to use any magnification like binoculars or a telescope to look right at the Sun. (Of course, a magnified image might be projected onto a sheet of paper for safe viewing.) When the sun gets closer to the horizon, the green rim is becoming fainter because of atmospheric reddening.Green and red rims by Andy Young. According to the above it is probably correct to conclude that although a green rim is present during every sunset, a green flash is more rare because it requires a mirage to be present. References See also *Mirage of astronomical objects External links *A Green Flash Page, Andrew T. Young's page with comprehensive explanations and simulations. *Green Flash - Atmospheric Optics, explanations and image gallery, Les Cowley's Atmospheric Optics site. *A Green Flash from Astronomy Picture of the Day, NASA. Gallery Image:Green flash gallerey.PNG| green flash bg:Зелен лъч cs:Zelený záblesk da:Grønt glimt de:Grüner Blitz fa:اشعه سبز fr:Rayon vert it:Raggio verde he:הבזק ירוק ja:グリーンフラッシュ no:Grønne flekker nn:Grøne flekkar pl:Zielony promień pt:Brilho Verde ru:Зелёный луч fi:Vihreä välähdys sv:Gröna blixten Category:Atmospheric optical phenomena